Two candidates working on the same project?

Is it possible for two PhD candidates to work on the same project in the same lab? But maybe different signalling pathways/mechanisms? Or is it a case of supervisor/lab head knows best and students just stick with what the boss tells you to do?

Why not, if they use different approaches, methods, or have different aims or a more or less different topic.

One must presume that long and short arguments contribute to the same end. - Epicurus...except casandra's that belong to the funniest, most interesting and imaginative (or over-imaginative?) ones, I suppose.